yup i noticed that too . All the clip shot from my Ex1 after render to Prores it look so Dark .
& All the clip export from final look more contrast & Darker than u see it in the Final cut Viewer .
that might be the Gamma from 1.8 to 2.2
Hope Apple will fix it soon .

Earlier versions of QT crushed the blacks of 5D2 files. Apple fixed that, but messed up the gamma. (It was too bright.) My gut feel is that the QT10 version is correct.

I was able to see the problem clearly in Vegas using the histogram tool. With QT7.6, you could see gaps and bumps on what should have been a smooth curve. The "look" is unimportant. The thing we want is to have all 256 levels from the camera to be unique. We can then grade in post with the best results possible.

I opened the attached JPEG, but couldn't see the histogram problems in either case. I believe that the scaling and JPEG encoding is smooshing the bits around.

Try saving a 1080p TIFF or other uncompressed format from your versions of QT. Look at the histograms with a good tool. (Photoshop's histogram might work, but it's really small.) To confirm things, you should see some inconsistencies with the QT7 photo. Once confirmed, see if the QT10 version is without gaps.

Apple finally went to a standard gamma of 2.2 in OS10.6 after years of assuming mac users would be using a 1.8 gamma (the default up to 10.5).

FCP and quicktime used to apply a gamma change to video content to simulate what a PC / TV viewer would see, effectively darkening the footage. If like me you used a 2.2 gamma then FCP and Quicktime would still apply the darkening making the video look darker than it really is which was the cause of so many "why does my video look washed out after I export it" questions.

So with the latest FCP and Snow Leopard we're now seeing it as it should look, without any simulation. As far as I'm concerned it's great, they should have done it years ago.

I'm not sure this is the reason for your issue though, I'm just wondering if your running QT7 on Snow Leopard?